In your case, mixing present and past tense is acceptable.
You are conjoining two seperate sentences together. It is possible for the left-hand sentence to be written in a different tense from the right-hand sentence.
Sharon may be irrational, but it was not completely her fault.
Sharon may be irrational. However, it was not completely her fault.
For the specific example, you provided, we can replace the word
but with the string
but behaves in English like the word
and behaves in mathematical logic.
The word "and" is like a plus sign (
+) in mathematics. For example, when mathematicians write
2 are considered to be two seperate things.
Likewise, you have two sentences, written in two different tenses. You have conjoined the two seperate sentences together.
Given any two grammatcially sentences
[Y] the string
[X] and [Y] is also a grammatically-correct, fully-formed, sentence. Sentences
[Y] are not required to be in the same tense (past, present, future).
Words such as "and" and "but" are somtimes used to concatenate two seperate sentences together.
I say "somtimes," rather than" always," because it depends on whether the word "but" or "and" are used at a high level or low level.
A word is used at a "high level" if that word apoears close to the root node when representing an English sentence in a tree diagram.
The word "and" is used at its highest level when it would be acceptable to replace the word
. (a period followed by a space).
Consider the following strings. Example 2 is inrentionally grammatically incorrect.
The house was large and
old. (Ex 1)
The house was large .
old. (Ex 2)
The house was large and
the house was old. (Ex 3)
"The house was large. The house was old." (Ex 4)
Like mathematics, the English language has a distributive property.
2*(4+9) = (2*4) + (2*9)
In the sentence
the house was large and old the string
The house was is distributed across the phrase
large and old.
Somtimes the word "and" is deeply nested inside of an English sentence. This is analogous to how a plus sign can be deeply nested inside of a mathematical formula. Other times, the word "and" is used at a high level (it is used close to the root of the sentence).
When the word "and" is used at a high level, it has a special property. When "and" is replaceable by period-space, then the operand sentences are allowed to be in two different tenses.
Sharon may have been irrational, but (logical-and) it was not completely her fault.
Sharon may have been irrational. It was not completely her fault.